[PLT-4135] Expose User.last_login_at in the Python SDK

[pawelkozik-labelbox]

https://labelbox.atlassian.net/browse/PLT-4135

Context

• Airbnb needs last-login information via the SDK to identify inactive users (e.g. no login in 90+ days)
• GraphQL already exposes User.lastLoginAt (PLT-3309); the Python SDK did not map or request that field
• Customers currently have user.created_at / user.updated_at but no last-login attribute on User

In scope

• Expose user.last_login_at on SDK User objects returned by the usual user APIs (get_user, get_users, org users, etc.)
• Value is a UTC datetime when known, or None until the user has logged in after last-login tracking was enabled
• Keep Issue/Comment user fragments in sync so nested User objects still hydrate correctly

Out of scope

• Account lockout / enforcement based on last login
• Changing how or how often last login is recorded on the backend

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Note

Low Risk
Additive SDK field mapping and fragment updates with no auth or enforcement logic changes.

Overview
Adds user.last_login_at on SDK User objects so customers can read last session time (login or workspace switch) for inactivity workflows. The value is a UTC datetime when the API provides it, or None until the user has logged in after last-login tracking was enabled.

Issue/comment GraphQL user fragments now request lastLoginAt so nested User instances on issues and comments hydrate the new field consistently. Integration and unit tests cover the attribute and mock payloads.

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